1. Field of the Invention
This invention relates to methods of and apparatus for use in decoding digital data which includes synchronizing codes and address codes. More particularly, but not exclusively, the invention relates to the decoding of audio and/or video data in digital form.
2. Description of the Prior Art
In the case, for example, of a video signal which has been put into digital form by sampling an incoming video signal and pulse code modulating the resulting samples to form data words, it is usual to assemble the data words into blocks. Associated with each data block is a synchronizing code and an address code, the address code including a block address. The resulting serial digital data may then be transmitted over a signal path or recorded and reproduced using a digital video tape recorder.
On reception or reproduction the digital data is supplied to a decoder. The functions of the decoder include identification of the synchronizing code and decoding of the address code. Identification of the synchronizing code enables the position of the address code and associated data blocks in the serial stream to be determined. The decoding of the address codes enables the data blocks to be attributed to their correct positions in the video field or frame. It is usual to provide the decoder with a flywheel circuit which, once locked to the incoming synchronizing codes, will continue to generate the synchronizing and address codes during any short break in the incoming synchronizing and address codes and caused, for example, by drop-out in a digital video tape recorder. The provision of such a flywheel circuit does not significantly reduce the need to achieve a high probability of correctly identifying incoming synchronizing and address codes; not least so that the flywheel circuit can be quickly and accurately locked.
In addition to total loss of incoming data for short periods due to drop-out on reproduction from a digital video tape recorder, recording and/or reproduction or any transmission or processing of the data will inevitably result in some random errors. So far as the data words in a digital video signal are concerned, it is now usual to use error correction codes, some of which are capable of achieving very high levels of correction of random errors. As a result, circumstances now arise where the largest source of error in the decoded signal is the failure of the decoder correctly to identify the synchronizing and address codes in the presence of random errors. This indicates that some form of protection against random errors in the synchronizing and address codes is required, but it is also important that the protection is obtained without an excessive overhead in the form of redundant data included solely to provide for error correction.